TWI317668B

When a laser beam is used to carryout weld-repairing in water, such troubles, as defective welding caused by weld metal being blown off when residual water is ejected, is limited or prevented. A protection plate (3) is welded to a metal structure (2) in water so that the protection plate (3) covers the surface of the structure (2) including a portion to be repaired. The welding is carried out such that the periphery of the protection plate (3) is welded in water by laser welding while inactive gas (10) is supplied to portion to be welded to seal the portion to be repaired by the protection plate (3). Welding is carried out while the inactive gas (10) is supplied to the portion to be welded coaxially with a laser beam (7) during the laser welding. The plate (3) is provided therein with an opening in advance so as to release vapor produced from water remaining during welding between the protection plate (3) and the structure (2) and the opening is sealed after the periphery of the protection plate (3) is welded